Steelmaking

ABSTRACT

In a method for controlling the final carbon content of a steel produced by an oxygen steelmaking process, the weight of the molten steel in the vessel is continuously monitored during the process by means of load cells. The load cells may be placed under the vessel support structure or alternatively within the vessel support structure with trunnions which support the vessel disposed to rest on the vessel. It is found that a minimum weight point occurs in the charge consistently at the same carbon content during steelmaking. The detection of this point is used in the establishment of a relationship between oxygen added after this point and the final carbon content of the steel. Providing that the initial charge weight is fairly constant and the amount of oxygen added after the minimum weight point is known, the relationship, conveniently in the form of a calibration curve, may be used to predict or control the final carbon conent of the steel.

United States Patent [1 1 Greneell et al.

Nov. 20, 1973 STEELMAKING [75] Inventors: Hugh Willmot Greneell; David James Bowen, both of Swansea, Great Britain [73] Assignee: British Steel Corporation, London,

England [22] Filed: Mar. 2, 1972 [21'] Appl. No.: 231,233

Primary ExaminerL. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg AttorneyLeo A. Rosetta 57 ABSTRACT In a method for controlling the final carbon content of a steel produced by an oxygen steelmaking process,

the weight of the molten steel in the vessel is continuously monitored during the process by means of load cells. The load cells may be placed under the vessel support structure or alternatively within the vessel support structure with trunnions which support the vessel disposed to rest on the vessel. It is found that a minimum weight point occurs in the charge consistently at the same carbon content during steelmaking. The detection of this point is used in the establishment of a relationship between oxygen added after this point and the final carbon content of the steel. Providing that the initial charge weight is fairly constant and the amount of oxygen added after the minimum weight point is known, the relationship, conveniently in the form of a calibration curve, may be used to predict or control the final carbon conent of the steel.

3 Claims, 6 Drawing Figures Patented Nov. 20, 1973 4 SheetsSheet 5 MMRDZE g NEE QZEIMK @m m E w Q m m mbmommmomm v GI Patented Nov. 20, 1973 3,773,497

4 Sheets-Sheet 4 MINUTES FROM START OF BLOW I FIG. 5. Q: k 400 I IL I is I |\M/N/MUM WEIGHT POINT 553 300 AT 029% III" cu. FT. 0F OXYGEN MIN/MUM WE/GH I I I I I I I I I I l l CARBON CONTENT REMAINING IN STEEL STEELMAKING This invention relates to oxygen steelmaking and is particularly concerned with controlling carbon content in the molten steel at the end of the refining process.

The control of the carbon content at the end of the oxygen blow in oxygen steelmaking processes has been a problem for many years. Various methods are well known in the art for attempting to predict end point and thereby control the carbon content both during and particularly at the end of the blow. These methods include studying the flame issuing from the mouth of the converter for change in flame colour, exhaust gas analysis, molten steel temperature measurement and acoustic analysis of the metal in the converter, all of which are indicative of composition changes and particularly of carbon content change, in the molten steel. These methods do, however, all suffer from the common disadvantage in that while they enable carbon content to be monitored at the time of measurement, an assessment of rate of carbon change or alternatively prediction of carbon end-point, that is to say carbon content of the finished steel heat, is extremely difficult. The prior processes also are relatively inaccurate and it is an object of the present invention to produce an improved method of carbon end-point determination.

The present invention is based on the discovery that an accurate and rapid prediction of carbon end-point may be made by the technique of continuously monitoring the weight of the converter or furnace vessel charge during the refining blow up to and beyond a minimum weight point.

It has been found that after the minimum weight point, carbon loss from the melt is related to the quantity of oxygen injected into the converter or furnace vessel. Once this relationship has been determined by a conventional calibration test run for a particular converter or furnace vessel, the carbon end-point can be accurately predicted and controlled by injecting the calculated quantity of oxygen into the melt after the minimum weight point condition. The term converter is applied herein to any plant in which steel is refined by any process involving oxygen injection including processes in which other materials such as fuels are injected into the vessel.

According to one aspect of the present invention, a method for controlling the final carbon content in a steel making process including oxygen injection, comprises detecting the minimum weight condition of the steel charge arising from oxygen injection and continuing oxygen injection after this condition for a period enabling the injection of a quantity of oxygen which is selected from a previously established relationship and which is effective to reduce the carbon content to the required level from that at the minimum weight condition.

Preferably the minimum charge weight is detected by monitoring the weight of the vessel in which the charge is held. The weight of the charged vessel conveniently is monitored by one or more load cells, well known in the art, which may be disposed either upon the vessel itself or more suitably are coupled to the vessel support structure.

According to another aspect of the present invention an apparatus for controlling the carbon content by steel in a steel-making process involving oxygen injection comprises a structure for supporting the vessel in which the steel is to be refined together with means coupled to the support structure and sensitive to the weight changes in the molten steel charge within the vessel, whereby to enable the minimum weight condition of the charge to be detected and permit the injection after this condition of a selected quantity of oxygen effective from previously established relationship to reduce the carbon to the required level.

The weight of the charged vessel conveniently is monitored by one or more load cells, well known in the art, which may be disposed either upon the vessel itself or more suitably coupled to the vessel support structure. In the case where the vessel is conventionally supported upon pedestals the load cells are, conveniently, disposed beneath the vessel support pedestals supporting the vessel. Alternatively the vessel may be supported on the pedestals by way of trunnions and the loads cells may be disposed beneath the trunnions.

The injection of the oxygen is, ideally, performed by means of a lance disposed above the melt although alternative methods of injection such as bottom blowing may be employed.

Suitably, the oxygen injection rate is maintained constant once the minimum point condition has been attained by the lance height and rate of oxygen injection may both be varied, if desired, before the minimum weight point condition has been attained so that refining rate and the conditions of refining, for example, slag formation rate and oxidation rate may be varied. Such consistency of operation as has been mentioned assists to enable the carbon end-point content to be controlled with great accuracy.

Output signals produced from the load cells, which are indicative of the weight of the charged vessel, are conveniently monitored by means of an electrical bridge circuit which is sensitive to output changes in the load cells caused by corresponding changes in the weight of the vessel. However alternative forms of measuring load cell output may be used.

Embodiments of the invention will be particularly described by way of example with reference to the accompanying drawings in which:

FIG. 1 is a front view of a weighing assembly according to one embodiment of the invention;

FIG. 2 is a side view of one of the two weighing assemblies necessary in a further embodiment of the invention;

FIG. 3 is a plan view of the weighing assembly as shown in FIG. 2;

FIG. 4 shows the composition changes occurring in the steel charge during refining;

FIG. 5 illustrates the typical weight change which occurs in the converter vessel during the last half of the refining process inoxygen steelmaking.

FIG. 6 is a typical curve showing the relationship between oxygen content injected into the converter vessel and the carbon end-point content obtained by this injection.

FIG. 1 shows a typical converter assembly comprising a converter vessel 10 having trunnions ll supported in trunnion bearings 12 within a pedestal framework indicated generally at 13. The vessel 10 has an upper opening to receive molten metal from, for example, a blast furnace and, if desired, scrap iron or scrap steel may also be added to the molten metal charge. The charge is refined by oxygen injected from a lance (not shown) lowered into the upper opening and terminating short of the vessel surface in this embodiment. After refining, the molten steel is teemed into a suitable mould by tilting the vessel about the trunnions 11.

In one convenient arrangement of the present invention load cells 14 are disposed beneath adjacent pedestals supporting the vessel 10 as shown in FIG. 1. Each load cell 14, consists essentially of a precision steel billet to which is intimately bonded a foil strain gauge sensitive to tensile or compressive load, induced by the weight of the vessel 10. By connecting these strips into an adjacent Wheatstone Bridge circuit (not shown) an electrical correlation can be obtained between the bridge output and the applied load caused by the weight change occurring in the converter during the refining process.

Converter pedestals 13 are right-angled for stability and consist of substantially vertical beams 15 and substantially horizontal beams 16 which are braced for added strength about inner corners 17 at which the beams are joined. The pedestals 13 are movable about pivots 18 to which the horizontal beams 16 are attached at their ends and the pivots 18 are anchored to the stage floor 19. Two load cells 14 are placed separately beneath each of the horizontal beams 16 of the converter pedestals 13 at or near the outer corners, the converter vessel trunnions 11 being placed into the trunnion bearings 12 which are located near the top of the vertical beams 15 of the pedestals 13. The converter vessel 10, together with its charge and the pedestals 13, is thus able to float upon the load cells 14 to provide effective charge weight transfer.

In an alternative arrangement shown in FIGS. 2 and 3 the load cells 20 are disposed beneath trunnions 21, to which the converter vessel (not shown) is attached. In this arrangement the top section of one of the two converter trunnion pedestals 22 is provided with a rectangularly shaped closely machined slot 23 which is defined by a horizontal base 24 and two vertical walls 25. The slot 23 is open to atmosphere and traverses pedestal 22 from side wall 26 to side wall 27, the side wall 27 being nearest to the converter vessel. Four load cells 20 are sited in the base 24 of slot 23 and a cuboidal shaped trunnion holding framework or cage 28 is inserted into slot 23 to rest on the load cells 20. The cage 28 is of smaller cross-section than the distance between the vertical walls of the slot 23 and is provided with four hardened precision cylindrical steel rollers 29 which are disposed within recesses in cage 28. Two rollers 29 are contained within either of the sides 30 of the cage 28 and revolve over walls 25 enabling the cage 28 to move vertically within slot 23. Trunnion bearing 31 is inserted into a cylindrical hole extending through the centre of opposing sides 32 of the cage 28, and encloses trunnion 21 supporting the converter vessel. A similar assembly as that described is provided at the top of the adjacent converter pedestal support. Signal variation from the load cells caused by the change in weight of the vessel are monitored and used to determine the minimum weight point.

During oxygen refining a number of composition changes occur in the steel as shown in FIG. 4. Carbon is removed continuously throughout the refining blow leaving the converter as either carbon monoxide or carbon dioxide so that a resultant charge weight loss occurs equivalent to the loss of carbon. Other elements, chiefly manganese, silicon and phosphorus, migrate from the metallic portion of the bath into the slag and appear mainly as oxides. The oxidation of these elements results in a charge weight gain. However, the largest single contribution to any charge weight increase is from the oxidation of iron into the slag, and it is the balance of this rate against the rate of carbon removal which determines the end point in the present invention.

FIG. 5 shows a typical weight change occurring in the charge during the last half of the refining process by which time silicon removal into the slag is substantially complete. In the Figure, line AB represents the period when iron oxides in the slag have been reduced to a minimum by the chemical action of carbon contained in the molten bath, and are starting to increase again as the carbon content, which is decreasing, is not able to reduce the iron oxide. Line BC represents the further reduction of carbon. Line CD represents the final stages of the refining process when the formation of iron oxide is greater than the rate of carbon removal so that the charge weight increases. Point C represents the minimum weight point which consistently occurs at a charge carbon level of about 0.29 percent by weight if a stable lance practice is followed. This point can be established by continuously monitoring the weight of the charged converter vessel during refining but may vary from vessel to vessel and with different refining practices.

According to the present invention the end-point carbon content desired can be predicted or selected by referring to a calibration curve such as that shown in FIG. 6 in which oxygen as ordinate is plotted against carbon end-point as abscissa. This curve is the result of a number of experimental trials wherein differing known quantities of oxygen are injected in the molten charge after the minimum weight of the charge has been attained. Once this curve is available it can be used by extrapolation or interpolation to quantify the amount of oxygen necessary to reach the desired carbon endpoint and therefore control the carbon end-point by injecting that quantity of oxygen into the charge before turndown. The quantity of oxygen injected into the charge can be determined by direct measurement of gas flow by conventional means or by controlling the time of flow given a known and constant flow rate. Alternatively the quantity of oxygen injected into the charge can be determined by monitoring the weight of the charge, as previously outlined, given the known relationship between the change in the weight and the quantity of oxygen injected.

We claim:

1. A method for controlling the final carbon content of a molten steel charge which comprises:

a. determining the relationship between the carbon content of the charge and the amount of oxygen injected into the charge after the minimum weight point of the charge has been attained? b. injecting oxygen into the steel charge and monitoring the weight of the charge during the oxygen injection; and

c. after the weight of the charge has reached a minimum level, injecting a quantity of oxygen which is selected from the relationship established in (a) into the charge to reduce the carbon content of the steel to a required value from that at the minimum level.

2. The method of claim 1, in which the minimum charge weight is detected by monitoring the weight of the vessel in which the charge is held.

3. The method of claim 2, in which the weight of the charged vessel is monitored by one or more load cells.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,773,497 Dated November 20, 1973 Inventor) Hugh Willmot Granfell and David James Bowen It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the title page the name of one of the inventors Hugh WillmotGrenfell, is spelled incorrectly.

Signed and sealed this 9th day or April 19m.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. Atte sting Officer C. MARSHALL DANN Commissioner of Patents USCOMM'DC 6376-P69 U.S. GOVERNMENT PRINTING OFFICE 2 i969 O-366-33l F OHM PO-105O (10-69) 

2. The method of claim 1, in which the minimum charge weight is detected by monitoring the weight of the vessel in which the charge is held.
 3. The method of claim 2, in which the weight of the charged vessel is monitored by one or more load cells. 